The present invention has been developed in connection with a tubing blow-out preventer ("BOP") having a ball as a valve bore sealing mechanism. The invention will be described in connection with that particular form of valve. However, it is anticipated that the invention will have application to other valves, including other forms of plug valve and gate valves, in situations where controlled pressure equalization may be desirable.
The tubing BOP is a piece of equipment brought to a well in connection with service rig operations. It is mounted to the upper end of the tubing string projecting from the wellhead. It commonly has a truncated ball, having a through bore, which can be rotated through 90.degree. to open or close the bore of the valve.
At the end of the working day, it is usual practice for the service rig crew to close the BOP to seal the tubing string. In some cases, considerable pressure (e.g. 2000 psi) can build up overnight in the tubing string. This pressure acts to press the ball upwardly against the upper seal ring of the valve housing. When the crew arrive the next day, the ball can be very difficult to turn. In many cases, a line and pump have to be tied to the top outlet of the BOP and pressure applied to the outlet side of the ball, to free it. This is time-consuming to carry out. Some inexperienced personnel don't understand the problem or what to do about it. This can result in delay until an experienced person arrives to organize pressure equalization.
There is therefore a problem of long standing for which no practical solution has been provided, as far as applicant is aware. It is the object of the invention to provide such a solution.
At this point, it is useful to describe a conventional tubing BOP having a ball as its bore sealing mechanism, such as the BOP, illustrated in FIG. 1. It comprises:
A tubular housing comprising a lower body and an upper sub. The body is internally threaded at its lower end for connection with the tubing. The body and sub are threadably connected. The sub is internally threaded at its upper end; PA1 The housing side wall forms a through bore. This bore consists of an inlet bore and ball chamber formed by the body and an outlet bore formed by the sub; PA1 The housing includes lower and upper seal rings which bracket the ball. Each seal ring forms an annular seal area against which the ball seals; PA1 The ball has flat sides. A stem (not shown) is secured to one side of the ball for turning it; PA1 The ball forms an axial through bore extending parallel to its sides, for connecting the inlet and outlet bores of the housing; PA1 The seal rings carry external O-rings for sealing against the internal surface of the housing; PA1 A clearance is formed between the outer surface of the ball, external of the annular seal areas, and the internal surface of the housing; and PA1 A spring is positioned between the base of the lower seal ring and the housing, for biasing the lower seal ring and ball tightly against the upper seal ring, to preserve the annular seals.
The present invention is an improvement to a conventional valve, of which a tubing BOP, having a ball sealing mechanism, is representative. This improvement will now be described.